1. Field of the Invention
The present invention generally relates to a magnetic tape memory device, and particularly relates to a method of and an apparatus for positioning a magnetic head relative to a magnetic tape in a direction along the width of the magnetic tape in a magnetic tape memory device which has a rotary head.
2. Description of the Prior Art
In computers, for example, the magnetic tape is used as an additional large volume memory media or as a backup memory media for copying information stored in memory devices such as hard-discs. A magnetic tape is wrapped around a pair of tape reels which is contained in a data cartridge, and a torque roller for rotating the pair of reels is provided on the front face of the data cartridge. When the data cartridge is inserted into a magnetic tape memory device, the magnetic tape memory device rotates the pair of reels by conveying torque through the torque roller. The magnetic tape is driven just inside the front face of the data cartridge, and the recording or reproducing of information is carried out by a magnetic head which is positioned so as to touch the magnetic tape.
Regarding such a memory device, there are methods in which a rotary head is used instead of a fixed conventional head in order to increase the memory volume, and one of these methods is a semi-circle scanning method.
FIGS. 1A and 1B show the semi-circle scanning method. As shown in FIG.1A, the recording and reproducing of information is carried out by rotating a rotary head 12 with magnetic heads 12b mounted on the perimeter of a head holder 12a and by driving a magnetic tape 11 in a direction A. In recording and reproducing information, the magnetic heads 12b touch the surface of the magnetic tape 11.
Since the magnetic heads 12b revolve around the center of the head holder 12a, record tracks 11a formed on the surface of the magnetic tape 11 have a shape of an arc. Thus, when reproducing information, the positioning of the rotary head 12 relative to the magnetic tape 11 has to be precise in order to obtain a correctly reproduced signal. To this end, the position of the rotary head 12 relative to the direction along the width of the magnetic tape 11 is strictly controlled by using a servo method.
A servo method requires appropriate data as a basis for control. This data is provided as servo data P, Q, and R as shown in FIG. 1B, where the servo data Q and R are recorded near the edges of the magnetic tape 11, and the servo data P is recorded at the center. The servo data P, Q, and R are recorded such that the distance between P and Q and the distance between P and R are the same. When reproducing information, the rotary head 12 is controlled to be positioned in a manner which satisfies the two conditions described below. First, the time distance between two signals reproduced from the servo data P and Q is the same as the time distance between two signals from the servo data P and R. Second, the address numbers of the servo data P, Q, and R are all the same. Here, the address numbers of the servo data P, Q, and R are recorded at the time of recording information, and are all made equal within one track.
FIGS. 2A and 2B show a schematic illustration of a rotary head unit 10. In the rotary head unit 10 of FIG. 2A, the magnetic heads 12b are provided on the perimeter of the head holder 12a, which is rotated by a head motor 13. The head motor 13 is mounted on a supporting base 14.
Tape guides 15a and 15b which position the magnetic tape 11 are provided on both sides of the head holder 12a. Supporting plates 16a and 16b (not shown) are provided on the sides of the supporting base 14 at approximately the center thereof. The supporting base 14 can rotate about rods 17a and 17b (not shown) in a direction B-C (a direction along the width of the magnetic tape 11). Also, the supporting base 14 is provided near the back end thereof with a voice coil unit 18 to control the movement of the supporting base 14 in the direction B-C.
The positioning of rotary head 12 relative to the magnetic tape 11 is carried out by controlling electric current provided for the voice coil unit 18 on the basis of the signals reproduced from the servo data P, Q, and R.
However, since the rotary head 12 is rotated around the rods 17a and 17b when its relative position to the magnetic tape 11 is being controlled, movement of the rotary head 12 in the direction B-C draws a shape of an arc as shown in FIG. 2B. Thus, the angle of the rotary head 12 relative to the surface of the magnetic tape 11 varies according to the position of the rotary head 12. This means that a reproduced signal has a proclivity to become unstable, which in turn means that the positioning of the rotary head 12 is also unstable.
Accordingly, there is a need in the field of magnetic tape memory devices for a method of, and an apparatus for, positioning a rotary head relative to a magnetic tape while keeping a constant angle between the rotary head and the surface of the magnetic tape.